The present invention relates generally to the field of sterilization and pasteurization, and more particularly to a method and apparatus for treatment of a fluid medium to inactivate biocontamination using a pulsed electric field (PEF).
High intensity pulsed electric field (PEF) processing is an effective means for treating a fluid medium, such as a liquid product (including, but not limited to, liquid foods and medicines), to inactivate biocontamination, such as microbes. PEF processing involves the application of a waveform comprised of high voltage pulses to a fluid medium flowing between a pair of electrodes. The pulses typically have a width in the range of 1 to 100 psec and a pulse frequency (i.e., pulse rate) in the range 1 to 10,000 pulses/sec. The high voltage pulses typically provide an electric field intensity in the range of 15 to 80 kV/cm. PEF processing is often recognized as superior to conventional thermal treatment of liquid foods, because it avoids or greatly reduces the detrimental changes of the sensory and physical properties of foods. At present, PEF processing is being used to improve food quality, such as extending the shelf-life of bread, milk, orange juice, liquid eggs, and apple juice, and to enhance the fermentation properties of brewers yeast.
A conventional PEF processing system includes a high voltage generator, a pulse generating circuit (e.g., an array of capacitors and inductors, and swithching devices) that produces high voltage pulses, and electrodes located in a treatment assembly. The electrodes receive the high voltage pulses, and produce an electric field. The liquid being treated flows between the electrodes, and thus receives exposure to the electric field.
There are several known methods for applying a PEF to a fluid medium. In this regard, the PEF may be applied in the form of exponentially decaying pulses, square wave pulses, bipolar pulses, instant-charge-reversal pulses, or oscillatory pulses. An exponential decay voltage wave is a unidirectional voltage that rises rapidly to a maximum value and decays slowly to zero. Square pulse waveforms are more lethal and more energy efficient than exponential decaying pulses. A square waveform can be obtained by using a pulse-forming network (PFN) consisting of an array of capacitors and inductors and switching devices. The instant-charge-reversal pulses are characterized by a positive voltage part (+ve) followed by a negative voltage part (xe2x88x92ve), with various widths and peak field strengths. There is no relaxation time between the positive and negative voltage parts. Bipolar pulses are pulses that reverse alternately, with a relaxation time between pulses. Research studies have indicated that oscillatory pulses seem to be the least efficient for microbial inactivation, because they prevent cells from being continuously exposed to a high intensity electric field for an extended period of time, thus preventing the cell membrane from irreversible breakdown over a large area.
The effectiveness of PEF processing is a function of several factors, including, but not limited to: electric field intensity, rise time of high voltage pulses for producing the pulsed electric field, number of pulses, pulse wave shape, treatment time (i.e., the product of the number of pulses applied and the duration of the pulses) processing temperature, characteristics of the target biocontamination, and the characteristics of the fluid medium (e.g., electrical conductivity of the medium, ionic strength of the medium, and viscosity and pH of the medium).
Several problems have been encountered in the prior art which have been observed to impair the effectiveness of PEF processing. For instance, prior art PEF processing systems generate high voltage pulses having long rise times. Furthermore, most prior art PEF processing systems have PEF treatment assemblies wherein the fluid medium flows past the electrodes in direct contact therewith. Consequently, over time, the electrodes are subject to corrosion due to contact with certain fluid mediums (e.g., corrosive liquids).
The present invention addresses these and other problems to provide an improved PEF processing system for treatment of fluid mediums.
In accordance with the present invention, there is provided an apparatus for exposing a fluid medium to a pulsed electric field, comprising (a) an energy storage circuit chargeable by a high voltage source; (b) switch means connected with the energy storage circuit, said switch means operable to form high voltage pulses; (c) a pulse compressing means for receiving the high voltage pulses and reducing a rise time for each of said high voltage pulses to produce a series of compressed high voltage pulses having a decreased rise front (i.e., shorter rise time); and (d) treatment assembly for receiving the fluid medium, said assembly comprising: (1) a dielectric flow tube providing a passageway for the fluid medium to travel through the treatment assembly, and (2) inductance means for receiving the series of compressed high voltage pulses, said inductance means producing a changing magnetic field in response to a change in voltage, which in turn induces an electric field, said electric field being pulsed in response to the compressed high voltage pulses, wherein said fluid medium passes through said pulsed electric field.
In accordance with another aspect of the present invention, there is provided a method for exposing a fluid medium to a pulsed electric field, comprising: (a) switching an energy storage element to form high voltage pulses; (b) compressing the high voltage pulses to reduce a rise time of each pulse, thereby producing compressed high voltage pulses; (c) exciting inductance means with the compressed high voltage pulses, wherein said compressed high voltage pulses produce a changing magnetic field, which in turn induces an electric field; (d) producing a series of electric field pulses in response to the high voltage pulses; and (e) exposing the fluid medium to the pulsed electric field.
In accordance with another aspect of the present invention, there is provided an apparatus for producing compressed pulses, comprising: (a) an outer conductor connected to ground; (b) an inner conductor connected with a switchable voltage source to receive one or more pulses of voltage having a first rise time; and (c) a magnetic material located between the inner and outer conductors.
In accordance with still another aspect of the present invention, there is provided an apparatus for exposing a fluid medium to a pulsed electric field, comprising: (a) a source of high voltage pulses; and (b) a treatment assembly including: (I) inductance means for receiving the high voltage pulses and inducing a pulsed electric field in response to the high voltage pulses, and (2) a passageway physically isolated from the inductance means, to pass the fluid medium through the pulsed electric field.
In accordance with yet another aspect of the present invention, there is provided a method for exposing a fluid medium to a pulsed electric field, comprising the steps of: (a) generating a series of high voltage pulses; (b) exciting inductance means with the series of high voltage pulses, wherein a pulsed electric field is induced in response to the high voltage pulses; (c) exposing the fluid medium to the pulsed electric field, wherein the fluid medium is physically isolated from the inductance means.
An advantage of the present invention is the provision of a PEF processing system that generates high intensity electric fields.
Another advantage of the present invention is the provision of a PEF processing system that generates electrical pulses having a short rise time.
Another advantage of the present invention is the provision of a PEF processing system that generates electrical pulses having an increased pulse rate.
Still another advantage of the present invention is the provision of a PEF processing system, including a treatment assembly, that physically isolates electrical components of the system from a fluid medium flowing therethrough.
Still another advantage of the present invention is the provision of a PEF processing system that reduces the required voltage level.
Yet another advantage of the present invention is the provision of a PEF processing system that provides an increase in throughput of fluids therethrough.
These and other advantages will become apparent from the following description of a preferred embodiment taken together with the accompanying drawings and the appended claims.